The present invention is directed to probe stations adapted for making highly accurate low-current and low-voltage measurements of wafers and other electronic test devices. More particularly, the invention relates to such a probe station having a guarding system for preventing current leakage, a Kelvin connection system to eliminate voltage losses caused by line resistances, and an electromagnetic interference (EMI) shielding system.
The technique of guarding to minimize current leakage during low-current measurements, the use of Kelvin connections for low-voltage measurements, and the provision of EMI shielding are all well known and discussed extensively in the technical literature. See, for example, an article by William Knauer entitled xe2x80x9cFixturing for Low-Current/Low-Voltage Parametric Testing,xe2x80x9d appearing in Evaluation Engineering, November, 1990, pages 150-153. See also Hewlett-Packard, xe2x80x9cApplication Note 356-HP 4142B Modular DC Source/Monitor Practical Application,xe2x80x9d (1987) pages 1-4, and Hewlett-Packard, H-P Model 4284A Precision LCR Meter, Operation Manual (1991) pages 2-1, 6-9, and 6-15.
In guarding applications, a conductor surrounding or otherwise closely adjacent to a low-current line or circuit is maintained at the same potential as the line or circuit to reduce leakage currents therefrom, so that low-current measurements can be made accurately.
Kelvin connections compensate for voltage losses caused by line resistances which would otherwise cause errors in low-voltage measurements. This is accomplished by providing a source line and a measurement line (also referred to commonly as xe2x80x9cforcexe2x80x9d and xe2x80x9csensexe2x80x9d lines, respectively) to an interconnection point (the Kelvin connection) which is as close to the test device as possible. A high-impedance voltmeter is connected to this interconnection point through the measurement line to accurately detect the voltage without any significant flow of current or resultant voltage drop in the measurement line. This avoids the error which would otherwise occur if the voltmeter were to detect the voltage through the source line, due to the voltage drop that occurs in the source line.
Probe stations have previously been used for conducting tests with guarding, Kelvin connection, and EMI shielding techniques. However the custom set-up of such probe stations required for guarding and Kelvin connection procedures is time-consuming and, in some instances, limited as to effectiveness. For example, in an article by Yousuke Yamamoto, entitled xe2x80x9cA compact Self-Shielding Prober for Accurate Measurement of On-Wafer Electron Devices,xe2x80x9d appearing in IEEE Transactions on Instrumentation and Measurement, Volume 38, No. 6, December, 1989, pages 1088-1093, a probe station is shown having a respective detachable triaxial connector mounted on the probe card and the chuck assembly which supports the test device. The intermediate connector element of a triaxial connector normally is utilized for guarding purposes. However the chuck assembly shown has only a chuck and a shield, with no separate guarding structure to which the intermediate connector element could be connected. Accordingly significant time-consuming alteration of such a station would be required to obtain both a guarded and shielded chuck assembly. The probes on the probe card, on the other hand, are both guarded and shielded; however there is no means of enabling each probe to be moved independently of the others in unison with its guard and shield to accommodate different contact patterns of test devices, thus sacrificing flexibility of the probe station. Also, there is no provision for Kelvin connections on the chuck assembly, which would require more than a single triaxial connector as shown.
Chuck assemblies are available which provide guarding and shielding components. For example, Temptronic Corporation of Newton, Mass. markets a thermal chuck assembly atop which is mounted an xe2x80x9cadd-onxe2x80x9d supporting surface for the test device, with a copper guarding layer interposed between the add-on surface and the underlying chuck assembly and insulated from each by respective sheets of insulating material. This structure permits a signal line to be soldered to the add-on surface, a guard line to be soldered to the copper guarding layer, and a ground line to be soldered to the underlying chuck assembly which can then serve as a shield. However such wiring requires time-consuming set-up, particularly if Kelvin connections are also required. Moreover, the use of sheet insulation to insulate the copper guarding layer from the add-on surface and the underlying chuck assembly fails to provide as low a dielectric constant between the respective elements as is desirable to minimize leakage currents in view of the low level of current to be measured.
With respect to probe stations that are designed to accommodate the measurement of low levels of current, a sensitivity threshold is normally encountered below which further improvements in current sensitivity are difficult to reliably achieve. In most commercial probe stations that are of such design, this sensitivity threshold is typically reached at about 20-50 femtoamps. However, improvements in device fabrication and in the capabilities of commercially available test instrumentation make it desirable to reduce the sensitivity threshold to a level reliably within the single digit femtoamp range.
A particular difficulty encountered in low-level current measurements is the excessive time required for measurement voltages to stabilize with reference to the device under test after a shift in voltage has occurred at the electrical input to the probe station. This problem of excessive settling time, as it is referred to, increases as the level of current under measurement is reduced. That is, due to the residual capacitance existing between spaced apart conductors in the region surrounding the immediate test site, a certain amount of time is required for the conductors that are in direct connection with the test device to fully charge or discharge to their desired voltages, and the time required will increase as the current through the device decreases. If the residual capacitance and the degree of input voltage shift are moderately large and if the level of current being measured is moderately small, the probe station operator can encounter settling times that are upwards of two or three minutes. Clearly, then, it is desirable that settling times be generally reduced in order to reduce overall measurement time, particularly where the device under test is a wafer containing large numbers of discrete devices, each of which may individually require low-level current testing.
In addition to settling effects, measurements of low level currents are also susceptible to error due to electrical discharge effects which occur because of the acceptance and release of charge by nonconductors in the region surrounding the immediate test site. At very low currents, these discharge effects can significantly distort measurement values and hence contribute to unacceptable levels of measurement instability.
The present invention solves the foregoing drawbacks of the prior probe stations by providing a probe station having integrated and ready-to-use guarding, Kelvin connection and shielding systems, both for individually movable probes and for the chuck assembly.
In further preferred embodiments of the invention, an improved guarding system is provided for accurate and rapid measurement of very low-level currents.
The chuck assembly of the present invention may in preferred embodiments thereof comprise at least first, second and third chuck assembly elements electrically insulated from one another and positioned at progressively greater distances from the probe(s) along the axis of approach between them. At least one detachable electrical connector assembly is provided on the chuck assembly having respective connector elements connected matingly to the first and second chuck assembly elements, respectively, so as to provide a ready-to-use guarding system requiring only the easy detachable connection of a guarded cable to the connector assembly for immediate operability.
Preferably, a second such detachable electrical connector assembly is also provided having its corresponding connector elements connected, in parallel with those of the first connector assembly, to the first and second chuck assembly elements so as to provide a ready-to-use guarded Kelvin connection on the chuck assembly which becomes immediately operable by the easy detachable connection of a second guarded cable to the second connector assembly. Thus one cable serves as a guarded source line and the other serves as a guarded measurement line.
Leakage currents in the chuck assembly are preferably minimized by the fact that the three chuck assembly elements are electrically insulated from one another by distributed patterns of dielectric spacers, rather than continuous dielectric sheets, so that large air gaps are provided between the respective chuck assembly elements to reduce the dielectric constant in the gaps between the elements.
In further preferred embodiments of the present invention, the second chuck assembly element is provided with respective upper, lower and skirting components to provide full guarding for the first chuck assembly element. In particular, respective surface portions on the upper, lower and skirting components extend opposite the upper, lower and peripheral surfaces, respectively, of the first chuck assembly element. Furthermore, a connector mechanism is provided that enables a nonzero potential to be established on the first chuck assembly element relative to ground, that is, relative to the outer shielding enclosure, and a substantially equal potential to be established on the second chuck assembly element.
In accordance, then, with a preferred method of use, the exemplary chuck assembly structure just described is energized via the connector mechanism so that the potential on the first element is effectively matched by a substantially equal potential on the second element whereby virtually no potential difference is developed in the region between the elements. As a result of this relationship and the arrangement of components of the second chuck assembly element, leakage current from the first chuck assembly element is reduced to virtually zero which enables low-level currents to be measured with increased sensitivity. Furthermore, with respect to low-level current measurements, settling times during startup and switchover phases of operation are reduced. That is, the second chuck assembly element, unlike the first, acquires or releases charge at a rate not limited by the large effective resistance presented by the device under test. Accordingly, the respective guarding components are able to achieve their full potential relatively quickly even though they are directly coupled capacitively to conductive surfaces of large area such as those on the outer shielding enclosure. The respective guarding components also serve as an effective barrier to stray radiation to the extent they are interposed between the element emitting such radiation and the first chuck assembly element. Therefore, relative even to the low levels of current being measured, the potential error or instability in each measurement is reduced to an insignificant level.
Individually movable probe holders are provided having not only ready-to-use guarded signal line cables and Kelvin connection cables, but also respective shields for the cables of each probe, the shields being movable independently in unison with each probe separately.
Where a line element of the connector mechanism that carries the signal is arranged exterior of its corresponding guard element, such as where it is separated out from the guard element for interconnection with another signal element, preferably a conductive guard enclosure is provided which surrounds the signal line element in interposed relationship between such element and the outer shielding enclosure. Furthermore, when a nonzero potential is established during low-level current measurement on the signal line element relative to ground, that is, relative to the outer shielding enclosure, preferably the connector mechanism is so connected to the guard enclosure as to enable a substantially equal potential to be established on the guard enclosure.
The signal line guarding system just described can thus be energized via the connector mechanism so that virtually no potential difference is developed between the signal line element and its surrounding guard enclosure. Hence, the level of leakage current flowing away from the signal line element is reduced to virtually zero which enables low-level currents in the system to be measured with increased sensitivity. Also, since there is a reduction in the combined area of the conductive surfaces to which the signal line element is capacitively coupled, less energy transfer and time is required for this line element to acquire its full potential, so that settling time is reduced. Moreover, if any transient shifts in electrical state should occur in relation to any nonconductor or conductor located outside the guard enclosure, this will have virtually no effect on the signal line element due to the effective barrier against radiation provided by the conductive guard enclosure, so that measurement instability is reduced.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.